Recipe Preparation

Place onion in a small heatproof bowl. Bring vinegar, sugar, salt, and 1/4 cup water to a boil. Pour over onion; stir to coat. Cover; let pickle in the refrigerator for at least 1 hour.

Whisk buttermilk, sour cream, and chives in a medium bowl until smooth. Stir in blue cheese. Drain pickled onion and stir into dip. Season with salt and pepper. DO AHEAD: Can be made 3 days ahead. Cover dip and refrigerate.